Industrial olefin oligomerization is most often catalyzed by organoaluminum compounds or transition metal compounds. The oligomerization of ethylene in particular gives a mixture of α-olefins. Of the α-olefins, 1-hexene has a high demand as a material for polyolefins, and high-selectivity processes for 1-hexene are desired. The only selective process that has been used in the industry is trimerization of ethylene using chromium compounds (Patent Document 1). This process affords approximately 8 kg of 1-hexene per 1 mmol chromium atom-hour under an ethylene pressure of 100 bar. However, it is preferred that a higher activity is achieved under a lower pressure so that the costs for pressure and catalyst in the production can be reduced. Further, very few techniques have been reported for the production of 1-hexene by trimerizing ethylene with transition metal compounds other than chromium compounds (Patent Documents 2 and 3, Non-Patent Documents 1 and 2).    Patent Document 1: U.S. Pat. No. 5,856,257    Patent Document 2: JP-A-2004-524959    Patent Document 3: WO 01/68572    Non-Patent Document 1: Journal of American Chemical Society, 2001, Vol. 123, pp. 7423-7424    Non-Patent Document 2: Journal of Organometallic Chemistry, 2004, Vol. 689, pp. 3641-3668